Heretofore, there have been known constant velocity joints of the ball joint type wherein the angle (joint angle) between two axes intersecting with each other is variable and wherein the constant or equality of velocity in the transmission of rotational movement is maintained between the two axes. The constant velocity joint is provided with an inner joint member, an outer joint member, a cage with ball retaining windows, and balls.
The inner joint member has its external surface taking a spherical surface and provided with plural first guide tracks extending in the axial direction thereof. The outer joint member is of a cup-shape having its internal surface taking a spherical surface and provided with plural second guide tracks of the same number as those of the first guide tracks, extending in the axial direction thereof. The cage is fit between the inner and outer joint members. The balls are retained in the ball retaining windows and are engaged with the first and second guide tracks for transmitting the torque of the inner joint member to the outer joint member.
The first and second guide tracks are formed to become shallower gradually from an opening portion side toward a bottom portion side of the outer joint member. The first and second guide tracks are constituted so that each of the balls is kept in contact with the first and second guide tracks at a constant contact angle over the whole length from the opening portion side toward the bottom portion side. Further, each of the first and second guide tracks is formed to have its cross-section constituted by two curved surfaces which intersect with each other at a track bottom center, and each of the curved surfaces is defined by a radius whose curvature is constant over from the opening portion side to the bottom portion side.
The term “contact angle” used herein means the angle that a contact center of the ball with the guide truck and the track bottom center of the guide track make with respect to a reference at the center of the ball. The term “contact center” herein means the point at which a major axis and a minor axis intersect with each other on a contact surface (contact ellipse) of an elliptical shape which is formed by the contact of each ball with each guide track. The term “major axis” means the axis which constitutes the longest portion in a lengthwise direction of the contact ellipse, and the term “minor axis” means the axis which constitutes the longest portion in a direction orthogonal to the major axis.
However, in the constant velocity joint of the foregoing prior art, the first and second guide tracks are formed to become shallower gradually from the opening portion side toward the bottom portion side (i.e., from the left side (opening portion side) toward the right (bottom portion side) as shown in FIG. 1). For this reason, as shown at the lower half in FIG. 1 and as shown in FIG. 5(a), when located at the bottom portion side of the outer joint member 113, each ball 101 occupies a deep track portion of the first guide track 102 of the inner joint member 112. Thus, the elliptical contact surface (hereafter referred to as “contact ellipse D” which is formed by the contact of the ball 101 with the first guide track 102 does not protrude from the first guide track 102.
However, the following problem arises when each ball 101 is located at the opening portion side of the outer joint member 113 as shown at the upper half in FIG. 1 and as shown in FIG. 5(b). That is, although the contact angle is the same as the contact angle (β) at the opening portion side and the curved surface is the same as the curved surface (K) at the opening portion side, the major-axis side radius of the contact ellipse D protrudes from the edge portion 102a of the first guide track 102, because the first guide track 102 of the inner joint member 112 is shallow. In this state, the ball 101 is liable to run on the edge portion 102a of the first guide track 102 thereby to exert an excess load on the edge portion 102a. Thus, the edge portion 102a is liable to chip off.
The present invention is made taking the foregoing circumstances into consideration, and it is an object thereof to provide a constant velocity joint which is capable of preventing each ball from protruding from a guide track therefor and of securing a sufficient contact area of the ball with the guide track.